Very nice find, congrats. I'm eager to see what's under the stickers of those keycaps.
cleaning the keycaps
Warm water and dish soap always worked well for mine.
which adapter should I get to make this board usable?
It's AT compatible. Eventually you might need an AT to PS/2 adapter if the cable ends with a DIN 5 plug.
There are just normally marked keycaps underneath, nothing special.
I'll clean the keycaps tomorrow
It's a DIN 5 plug, I already ordered the adapter.